1. Field of the Invention
This invention relates to a method for stimulating the production of fluids from earthen formations. More particularly, this invention relates to a method in which the permeability and porosity of a hydrocarbon-bearing formation containing sandstone and clay components are improved upon treatment of the formation with an aqueous solution of a mineral acid such as hydrochloric acid and a fluorine-containing acid or salt and having dissolved therein an oxyalkylated acrylamido alkanesulfonic acid polymer.
2. Description of the Prior Art
The technique of increasing the permeability of a subterranean hydrocarbon-bearing formation for the purpose of stimulating the production of fluids therefrom has long been practiced in the art. One such method commonly employed is known as acidizing which is widely utilized in treating subsurface geological formations, e.g., sandstone, limestone, dolomite, etc. In the usual well-acidizing procedure, a non-oxidizing mineral acid or mixture of acids such as hydrochloric and hydrofluoric is introduced into the well and under sufficient pressure is forced into the adjacent subterranean formation where it reacts with acid-reactive components, i.e., the siliceous materials, the carbonates, etc.
During the process passageways for fluid flow are created or existing passageways therein are enlarged thus stimulating the production of oil water, brines and various gases. If desired, the acidization may be carried out at an injection pressure sufficiently great to create fractures in the strata or formation which has the desired advantage of opening passageways into the formation along which the acid can travel to more remote areas from the well bore. The salts formed are extensively water soluble and are readily removed by reverse flow from the formation via the well bore.
There are, however, disadvantages attending the use of hydrochloric acid or other similar non-oxidizing mineral acids. For example, these strong aqueous acids have almost instantaneous rates of reaction with the acid-reactive components contained in the formation. The acid, therefore, necessarily spends itself in the formation immediately adjacent the well bore so little beneficial effect is realized at any great distance from the well bore within the formation under treatment. These strong aqueous acids also may cause channeling, cavitation and eventual collapse of the formation immediately adjacent the well bore due to excessively rapid action of the acid. In addition, subsurface equipment may be damaged severely by strong acid attack.
Treatment of sandstone formations by mixed hydrochloric-hydrofluoric acids has been used in the past as a means of removing damage caused by the presence of clays either originally present in the formation or introduced into the formation during drilling operations. The removal of such clays is accomplished by dissolution by reaction with the hydrofluoric acid: EQU 36HF+Al.sub.2 Si.sub.4 O.sub.10 (OH).sub.2 .fwdarw.4H.sub.2 SiF.sub.6 +12H.sub.2 O+2H.sub.3 AlF.sub.6.
Furthermore, the hydrofluoric acid component of the mixed acid will react with the sand and other siliceous minerals according to the following equation: EQU 6HF+SiO.sub.2 .fwdarw.H.sub.2 SiF.sub.6 +2H.sub.2 O.
The dissolution of both clays and siliceous material can materially increase the permeability and porosity of the formation in the vicinity of the wellbore and hence the production rate of the well. Although treatment of the formation in the vicinity of the injection wellbore usually results in an increase in the flow capabilities of the individual well under treatment, the response in production improvement may be only temporary.
It has been widely assumed that these mixed acid systems could be injected into a sandstone formation to dissolve clay at almost any depth from the wellbore, and that their reaction upon silica surfaces is so slow that little reaction takes place. A recent study (J. Pet. Techn., Vol. XXII, June 1970, p. 693) has shown that the reaction rate of mixed HCl-HF on clays is virtually instantaneous. Since clay is the mineral that usually causes most of the damage in sandstone formations, retarding the reaction rate of the acid mixture toward both sandstones and clays would be greatly beneficial. There are two types of mud damage: (1) Completion damage, which results from drilling mud permeating the pore spaces of the formation adjacent to the wellbore which is generally limited to a zone extending about one inch from the wellbore, and (2) Natural damage, which results from a reduction in virgin permeability as a result of swelling or migration of sensitive clays which may cause plugging of the formation flow channels. Natural damage is usually thought to exist to a dpeth of 2 or 3 feet from the wellbore. However, it can extend 7 or 8 feet or more from the wellbore. One can see that when treating formations with natural clay damage, the production increase realized is dependent upon the distance which the hydrofluoric acid can be pumped into the formation before being spent. Furthermore, as the depth of damage increases, the need for an acid mixture which will attack the formation at a greater distance from the wellbore becomes more desirable.
One method suggested to overcome the above-mentioned disadvantage of the fast reaction of the acid solution in the vicinity of the wellbore has been the use of "retarded" acids which consist, for example, of mineral acids and an additive which emulsifies the acid, a combination of which affects the acidization rate. Although such emulsified mixtures can be displaced into the formation before substantial reaction occurs, such compositions have the inherent disadvantage in that when the emulsion breaks and they do react, they usually react swiftly, often unpredictably with the result that problems of cavitation and channeling develop. More importantly, the use of such emulsified retarded acid has been limited to conventional acidization processes using hydrochloric acid alone, which has no utility in the dissolution of silica or clays.
The use of cross-linked copolymers of polyvinylpyrrolidone and polyacrylamides, polyurethanes, etc. to give materials which are insoluble in aqueous mineral acid solutions is described in U.S. Pat. No. 3,380,529 to Hendrickson. Such insoluble, cross-linked polymers are utilized as agents for partially plugging channels developed during acidization treatment in order to effect acidic attack at greater distances from the wellbore. In U.S. Pat. No. 3,434,971 a similar acidization process is described in which copolymer prepared by polymerizing acrylamide and N-vinylpyrrolidone in the presence of a cross-linking agent, such as N,N-methylenebisacrylamide, is disclosed. These copolymers are insoluble in the aqueous mineral acid solutions employed in acidization processes which utilize insoluble cross-linked copolymers and are distinctly different from the novel method of this invention in which a polymer soluble in aqueous mineral acid solutions is used.
It is therefore, the principal object of the present invention to overcome the defects of the prior art in acidizing fluid-bearing formations such as hydrocarbon-bearing formations, etc., by providing a method utilizing the novel acidizing composition of this invention in which the distance to which the acidizing composition penetrates the formation before becoming spent is extended, thus providing in-depth acidization.
Another object of this invention is to provide a method of acidization employing the novel composition of this invention which is effective in sandstone and clay-containing formations.
Another object of this invention is to provide a high viscosity acidizing fluid containing in solution an oxyalkylated acrylamido alkanesulfonic acid polymer which is stable over long periods of time in which the polymer constituent does not hydrolyze or otherwise decompose on storage, thus avoiding the simultaneous reduction in viscosity and effectiveness in acidization-in-depth operations.